The present invention relates generally to the positioning of printing surfaces or printing devices and specifically to the use of various calibration devices and methods for positioning of a printing device relative to print media, such as paper.
Multi-scan printing involves the use of a printing device smaller than the size of a piece of paper. Therefore, to print on the entire piece of paper, the printing device is moved relative to the piece of paper during the process of printing. Multi-scan printing provides many benefits, including low cost from the use of small printing devices. Also, very large pieces of paper can be imprinted by the use of multi-scan printing.
One difficulty in multi-scan printing involves relocating the printing device relative to the piece of paper from one printing swath to the next. The process of juxtaposing two swaths is called xe2x80x9cstitching.xe2x80x9d Stitching accuracy must be high for the printed image not to contain undesirable visible artifacts. Similarly, the use of multiple printing devices to obtain a multi-color printed image also requires the alignment of one printing device to another to avoid visible artifacts.
One approach to dealing with the difficulties in multi-scan printing has been the use of printing devices to create narrow swaths and, therefore, frequent stitching of the swaths. By the use of narrow swaths, it is possible to move the printing device relative to the piece of paper a known distance by the rotation of gears, preferably one rotation per swath. Printing swath widths in this type of multi-scan printing are typically less than one centimeter wide. However, this approach reduces printing efficiency by requiring many swaths to print an image.
A more efficient approach to multi-scan printing does involve the use of larger printing devices, such as printing devices capable of printing a swath of over 1 cm wide. Multi-scan printing involving wider swaths provides substantial benefit in increasing the speed of printing. However, one difficulty of this type of multi-scan printing involves the positioning of the printing device relative to the paper in order to provide high accuracy in stitching. One approach has been to use high accuracy encoders to establish a location of the printing device relative to the paper. High costs of such precise encoders have proven to be prohibitive in some applications. Furthermore, calibration of such encoders can be difficult. For example, while factory calibration procedures may initially calibrate the encoders, by the time a printing device is put in service in the field, the encoders may be out of alignment, resulting in poor stitching. Even if calibration can be maintained up to the time of initial use of the printing device, a printing device may experience a change in alignment characteristics during use due to changes of temperatures of various components involved with positioning the printing device relative to the piece of paper. Furthermore, a printing device will likely eventually require replacement. In any event, requiring the return of a printing device to the factory for calibration or replacement is typically undesirable.
The present invention recognizes a need in the art to provide the ability to precisely locate a printing device relative to a piece of paper while avoiding a need for expensive encoders. The present invention overcomes the difficulties of the prior art by the use of an optical sensor, preferably mounted to a printing device. The optical sensor is adapted to monitor marks on a piece of paper or on a paper handling surface configured to move the piece of paper.
According to one embodiment of the invention, a paper positioning system is provided having a paper-handling surface having marks intersecting an axis and an optical sensor configured to be located along the axis during advancement of the paper-handling surface and capable of detecting movement of the paper-handling surface by monitoring the marks, when the marks are sized or spaced non-uniformly long the axis with respect to each other.
According to another embodiment of the invention, an image forming system is provided having a paper-handling surface with non-uniform marks intersecting an axis and capable of moving a piece of paper in a direction substantially parallel to the axis, a carriage adapted for accommodating printing devices, mounted in slidable relation to the paper-handling surface to slide in the direction substantially perpendicular to the axis and substantially parallel to the paper-handling surface, an optical sensor mounted to the carriage and configured to be located along the axis during movement of the paper-handling surface and capable of detecting the movement of the paper-handling surface relative to the carriage by monitoring the marks.
According to another embodiment of the invention, a method of positioning paper for imprinting is provided including the steps of providing a paper-handling surface having non-uniform marks intersecting an axis, affixing the paper to the paper-handling surface and locating an optical sensor proximate to the axis such that the optical sensor can monitor movement of the paper-handling surface.
According to another embodiment of the invention, a paper positioning calibration system is provided having a printing device configured to imprint a piece of paper, an optical sensor mounted to the printing device and configured to monitor imprints on the paper, a controller adapted to receive data from the optical sensor and control movement of the printing device, the optical sensor and the paper. According to this embodiment of the invention, the printing device is adapted to form lines in at least two separate swaths parallel to the first axis which is substantially parallel to direction of travel of the printing device across the paper and perpendicular to a second axis which is parallel to direction of travel of the paper. Also, the optical sensor is located so as to detect at least one of the lines in each of the two separate swaths, and the controller is adapted to adjust the movement of the paper by detecting a relative position of one of the lines in each of the two separate swaths.
According to another embodiment of the invention, a method of paper positioning calibration is provided including the steps of providing a paper-handling surface, affixing a piece of paper to the paper-handling surface, locating an optical sensor and a printing device proximate to the paper, imprinting the paper with at least a first line oriented perpendicularly to a direction of travel of the paper relative to the printing device, moving the paper an intended distance in the direction of travel relative to the printing device, imprinting the paper with at least a second line substantially parallel to the first line, positioning the optical sensor simultaneously over the first and second lines, comparing a first distance between the first and second lines to an expected distance between the first and second lines based on the intended distance and determining a calibration value to cause the first distance to equal the expected distance.
According to another embodiment of the invention, a print head calibration system is provided having a first printing device configured to imprint a piece of paper with a first color, a second printing device configured to imprint the paper with a second color, an optical sensor mounted to the printing device and configured to monitor imprints on the paper, and a controller adapted to receive data from the optical sensor and control the first printing device, the second printing device, the optical sensor and a location of the paper. According to this embodiment of the invention, the first printing device is adapted to form a first line of the first color and the second printing device is adapted to form a second line of the second color an intended distance from the first line, wherein the first line and the second line are substantially parallel to the first axis which is perpendicular to a direction of travel of the paper. Furthermore, the optical sensor is located so as to detect the first line and the second line and allow the controller to determine the detected distance between the first line and the second line, and the controller is adapted to adjust an output of at least one of the first printing device and the second printing device to adjust an output of at least one of the first printing device and the second printing device to minimize the difference between the intended difference and the detected distance.
According to another embodiment of the invention, the method of print head calibration is provided having the steps of providing a paper-handling surface, affixing the paper to the paper-handling surface, locating an optical sensor and a printing device proximate to the paper, activating a first printing device to imprint the paper with at least a first line oriented perpendicularly to direction of travel of the paper relative to the printing device, activating a second printing device to imprint the paper with at least a second line an intended distance away from the first line and oriented perpendicularly to a direction of travel of the paper relative to the printing device, positioning the optical sensor simultaneously over the first line and the second line, comparing the intended distance to a detected distance between the first line and the second line and adjusting an output of at least one of the first printing device and the second printing device to minimize the difference between the intended distance and the detected distance.
According to a further embodiment of the invention, a printing device travel calibration system is provided having a printing device carriage configured to move along a first axis, an encoder configured to monitor a position of the printing device carriage along the first axis, a series of marks intersecting a second axis, wherein the second axis is substantially parallel to the first axis, an optical sensor mounted to the printing device carriage and configured to detect the marks, a controller adapted to receive data from the optical sensor and the encoder and control the location of the printing device carriage, wherein the controller compares an output from the optical sensor and an output from the encoder during movement of the printing device carriage along the axis and selects an encoder calibration value to adjust the output from the encoder to correspond to the output from the optical sensor.
According to another embodiment of the invention, a method of printing device travel calibration is provided having the steps of providing a printing device carriage configured to move along a first axis, providing a series of marks intersecting a second axis, wherein the second axis is substantially parallel to the first axis, monitoring movement of the printing device carriage along the first axis by the use of an encoder, detecting movement of the printing device carriage along the first axis by the use of an optical sensor mounted to the printing device carriage and in view of the marks and comparing an output of the monitoring step in an output of the detecting step to determine an encoder calibration value to correct the output of the monitoring step to correspond to the output of the detecting step.
According to another embodiment of the invention, a print head calibration system is provided having a first printing device configured to imprint a piece of paper with a first color and move along a first axis across the paper, a second printing device configured to imprint the paper with a second color and move along the first axis across the paper, an optical sensor configured to monitor imprints on the paper and a controller adapted to receive data from the optical sensor. According to this embodiment, the first printing device is adapted to form a first line of the first color perpendicular to the first axis and the second printing device is adapted to form a second line of the second color an intended distance from and parallel to the first line, and the optical sensor is located over the first line and the second line and obtains a detected distance between the first line and the second line. Also, the controller compares the intended distance to the detected distance to determine a calibration value for adjustment of at least one of the first printing device and the second printing device to minimize the difference between the intended distance and the detected distance.
According to a further embodiment of the invention, a method of print head calibration is provided having the steps of activating a first printing device to imprint a piece of paper with at least a first line oriented perpendicularly to a direction of travel of the first printing device, activating a second printing device to imprint the paper with at least a second line an intended distance away from the first line and parallel to the first line, detecting the detected distance between the first line and the second line by the use of an optical sensor and comparing the intended distance to the detected distance to determine a calibration value for adjustment of at least one of the first printing device and the second printing device to minimize the difference between the intended distance and the detected distance.